“Studying the past teaches us how today is different. The rate of CO₂ change today really is unprecedented,” said Kathleen Wendt, an assistant professor in Oregon State University’s College of Earth, Ocean, and Atmospheric Sciences and the study’s lead author.

“Our research identified the fastest rates of past natural CO₂ rise ever observed, and the rate occurring today, largely driven by human emissions, is 10 times higher.”

Carbon dioxide, or CO₂, is a greenhouse gas that occurs naturally in the atmosphere. When carbon dioxideenters the atmosphere, it contributes to warming of the climate due to the greenhouse effect. In the past, the levels have fluctuated due to ice age cycles and other natural causes, but today they are rising because of human emissions.

Ice that built up in Antarctic over hundreds of thousands of years includes ancient atmospheric gasses trapped in air bubbles. Scientists use samples of that ice, collected by drilling cores up to 2 miles (3.2 kilometers) deep, to analyze the trace chemicals and build records of past climate. The U.S. National Science Foundation supported the ice core drilling and the chemical analysis used in the study.

Previous research showed that during the last ice age, which ended about 10,000 years ago, there were several periods where carbon dioxide levels appeared to jump much higher than the average. But those measurements were not detailed enough to reveal the full nature of the rapid changes, limiting scientists’ ability to understand what was occurring, Wendt said.

“You probably wouldn’t expect to see that in the dead of the last ice age,” she said. “But our interest was piqued, and we wanted to go back to those periods and conduct measurements at greater detail to find out what was happening.”

Using samples from the West Antarctic Ice Sheet Divide ice core, Wendt and colleagues investigated what was occurring during those periods. They identified a pattern that showed that these jumps in carbon dioxide occurred alongside North Atlantic cold intervals known as Heinrich Events that are associated with abrupt climate shifts around the world.

“These Heinrich Events are truly remarkable,” said Christo Buizert, an associate professor in the College of Earth, Ocean, and Atmospheric Sciences and co-author of the study. “We think they are caused by a dramatic collapse of the North American ice sheet. This sets into motion a chain reaction that involves changes to the tropical monsoons, the Southern hemisphere westerly winds and these large burps of CO₂ coming out of the oceans.”

During the largest of the natural rises, carbon dioxide increased by about 14 parts per million in 55 years. And the jumps occurred about once every 7,000 years or so. At today’s rates, that magnitude of increase takes only 5 to 6 years.

Evidence suggests that during past periods of natural carbon dioxide rise, the westerly winds that play an important role in the circulation of the deep ocean were also strengthening, leading to a rapid release of CO₂ from the Southern Ocean.

Other research has suggested that these westerlies will strengthen over the next century due to climate change. The new findings suggest that if that occurs, it will reduce the Southern Ocean’s capacity to absorb human-generated carbon dioxide, the researchers noted.

“We rely on the Southern Ocean to take up part of the carbon dioxide we emit, but rapidly increasing southerly winds weaken its ability to do so,” Wendt said.

Additional coauthors include Ed Brook, Kyle Niezgoda and Michael Kalk of Oregon State; Christoph Nehrbass-Ahlesof the University of Bern in Switzerland and the National Physical Laboratory in the United Kingdom; Thomas Stocker, Jochen Schmitt and Hubertus Fischer of the University of Bern; Laurie Menviel of the University of New South Wales in Australia; James Rae of the University of St. Andrews in the United Kingdom; Juan Muglia of Argentina; David Ferreira of the University of Reading in the United Kingdom and Shaun Marcott of University of Wisconsin-Madison.

The increased performance of hybrids is generally known as hybrid vigour, or heterosis, and has been observed in many different plant (and animal) species. However, the heterosis effect no longer persists in the subsequent generations of these hybrids due to the segregation of genetic information. Thus, new hybrid seeds need to be produced every year, a labor-intensive and expensive endeavor that doesn’t work well for every crop.

So, how can the beneficial traits, encoded in the genes of hybrid plants, be transferred to the next generation?

Typically, our genetic material undergoes reshuffling during meiosis — a crucial cell division occurring in all sexually reproducing organisms. This reshuffling, due to random segregation of chromosomes and meiotic recombination, is important in generating novel and beneficial genetic configurations in natural populations and during breeding.

However, when it comes to plant breeding, once you have a great combination you want to keep it and not lose it by reshuffling the genes again. Having a system that bypasses meiosis and would result in sex cells (egg and sperm) that are genetically identical to the parents could have several applications.

In this study, Underwood and his team established a system, in which they replace the meiosis by mitosis, a simple cell division, in the most popular vegetable crop plant, the cultivated tomato. In the so-called MiMesystem (Mitosis instead of Meiosis) the cell division mimics a mitosis, thus sidestepping genetic recombination and segregation, and produces sex cells that are exact clones of the parent plant. The concept of the MiMesystem has previously been established by MPIPZ director Raphael Mercier in Arabidopsis and rice.

A breakthrough aspect of the new study is that for the first time the researchers harnessed the clonal sex cells to engineer offspring through a process they call “polyploid genome design.”

Usually, sex cells have a halved chromosome set (in humans, 46 chromosomes reduces to 23; in tomato 24 chromosomes reduces to 12) whereas the MiMe sex cells are clonal and therefore this halving of the chromosome set does not happen. Underwood and his team performed crosses that meant that the clonal egg from one MiMe tomato plant was fertilized by a clonal sperm from another MiMe tomato plant. The resulting tomato plants contained the complete genetic repertoire of both parents — and is thereby made up of 48 chromosomes. Hence all favourable characteristics from both hybrid parents are consolidated — by design — in one novel tomato plant.

Because of the close genetic relationship between tomatoes and potatoes, the team around Underwood believes that the system described in this study can be easily adapted for use in potato, the world’s fifth most valuable crop plant, and potentially other crop species.

In view of rising population figures and climatic changes, the development of high-yielding, sustainable, and stable varieties is crucial to securing the world’s food supply in the long term. Therefore, it is critical to cultivate plants that exhibit heightened disease resistance and stress tolerance. Innovative approaches to plant reproduction technologies are essential. The MiMe system and its application in polyploid genome engineering could be one promising avenue to tackle today’s agricultural challenges.

“We are really excited about the possibility of using clonal sex cells to carry out polyploid genome design. We are convinced this will allow breeders to untap further heterosis — the progressive heterosis found in polyploids — in a controlled manner,” says Charles Underwood.

“The tomato MiMe system we have established could also be used as a component of clonal seed production — synthetic apomixis — in the future. This could massively reduce the cost of producing hybrid seeds,” adds Yazhong Wang.

“The mind prioritizes remembering things that it is not able to explain very well,” said Ilker Yildirim, an assistant professor of psychology in Yale’s Faculty of Arts and Sciences and senior author of the paper. “If a scene is predictable, and not surprising, it might be ignored.”

For example, a person may be briefly confused by the presence of a fire hydrant in a remote natural environment, making the image difficult to interpret, and therefore more memorable. “Our study explored the question of which visual information is memorable by pairing a computational model of scene complexity with a behavioral study,” said Yildirim.

For the study, which was led by Yildirim and John Lafferty, the John C. Malone Professor of Statistics and Data Science at Yale, the researchers developed a computational model that addressed two steps in memory formation — the compression of visual signals and their reconstruction.

Based on this model, they designed a series of experiments in which people were asked if they remembered specific images from a sequence of natural images shown in rapid succession. The Yale team found that the harder it was for the computational model to reconstruct an image, the more likely the image would be remembered by the participants.

“We used an AI model to try to shed light on perception of scenes by people — this understanding could help in the development of more efficient memory systems for AI in the future,” said Lafferty, who is also the director of the Center for Neurocomputation and Machine Intelligence at the Wu Tsai Institute at Yale.

Former Yale graduate students Qi Lin (Psychology) and Zifan Lin (Statistics and Data Science) are co-first authors of the paper.